Method and device for estimating the transfer function of a channel for the variable transmission of a multicarrier signal

ABSTRACT

The method of estimating the transfer function of a varying transmission channel for a multicarrier signal (A) is performed on the basis of a frequency-domain digital signal (Y) corresponding to the received signal, and it comprises a step ( 6 ) of calculating the value of a first estimated transfer function (C 1 ) for the transmission channel. The method is characterized in that it further comprises: a step ( 8 ) of calculating the value of the autocorrelation function (R(C 1 )) of said first estimated transfer function (C 1 ) enabling the contribution from noise to be separated out; a step ( 10 ) of determining a noise-removal filter (F) from said autocorrelation function (R(C 1 )); and a step ( 12 ) of applying said noise-removal filter (F) to said first estimated transfer function (C 1 ) in order to deliver a second estimated transfer function (C 2 ) of the transmission channel from which noise has been removed. The invention is particularly applicable to receiving digital TV signals.

The present invention relates to a method and to apparatus for improvedestimation of the transfer function of a varying transmission channelfor transmitting a multicarrier signal.

In the context in particular of radio transmission, it is commonpractice to make use of modulation techniques that enable so-called“multicarrier” signals to be transmitted that enable information to betransmitted by multiplexing carriers at different frequencies.

For example, in the modulation techniques known as orthogonalfrequency-division multiplexing (OFDM) or coded OFDM (COFDM), binarydata is transmitted by being modulated on carriers of differentfrequencies.

For example, in the context of applying OFDM or COFDM to transmittingdigital TV information, the usable passband which may be about 8megahertz (MHz) with a spacing between carriers of 1 kilohertz (Hz)enables more than 8000 carriers of different frequencies to be used.

These signals are then subdivided in time into blocks that are known as“symbols” and that are of a duration that enables one period of thelowest carrier frequency to be sent, for transmission by radio.

Radio transmission leads to propagation irregularities due, amongstother things, to reflections and echoes induced by the environment, andto the fact that a plurality of transmitters are used.

Such transmission thus leads to the amplitude and the phase of each ofthe carriers in the transmitted signal being modified.

In order to model such modifications, the transmission channel isdefined in the form of a transfer function, such that the receivedsignal corresponds to the signal as transmitted and as transformed bysaid transfer function, and to which noise is also added.

In order to correct for the influence of radio transmission, referencecarriers known as “pilots” are introduced into the transmitted signal,which pilots are known in advance and make it possible at the receiverto estimate the modifications to which the signal has been subjected bybeing transmitted, and thus makes it possible to estimate the transferfunction of the transmission channel.

In order to perform this estimation, existing methods and apparatusesare based on the assumption that the transmission channel varies slowlyand continuously, so as to make it possible to use a plurality ofconsecutive channel estimates in order to distinguish between thetransfer function of the transmission channel and noise that isindependent of the channel.

Nevertheless, when variations in a transmission channel take placequickly and/or discontinuously, the above assumption no longer applies.

Specifically, for the application to receiving while on the move, orwhile in the presence of moving bodies (people, vehicles) close to thereceiver antennas and giving rise to varying reception conditions, theuse of existing methods and apparatuses lead to erroneous estimates ofthe transfer function of the channel, which in turn lead toinappropriate corrections that cause signal losses to occur.

The object of the invention is to solve that problem by defining amethod and apparatus for improved estimation of the transfer function ofa varying transmission channel.

The present invention thus provides a method of estimating the transferfunction of a varying transmission channel for a multicarrier signalincluding a plurality of known reference data items referred to aspilots, said method being implemented from a frequency-domain digitalsignal corresponding to the signal as received, digitized, synchronized,and transformed from a time base to a frequency base, and including astep of calculating the value of a first estimated transfer function forthe transmission channel on the basis of the pilots contained in saidfrequency-domain digital signal, the method being characterized in thatit further comprises:

-   -   a step of calculating the value of the autocorrelation function        of said first estimated transfer function enabling to separate        out the contribution from noise;    -   a step of determining a noise-removal filter from said        autocorrelation function; and    -   a step of applying said noise-removal filter to said first        estimated transfer function in order to deliver a second        estimated transfer function of the transmission channel from        which noise has been removed.

According to other characteristics of the method:

-   -   it is implemented periodically;    -   it is implemented continuously; and    -   said step of calculating a first estimated transfer function        comprises a substep of calculating the modifications to which        the pilots have been subjected in order to define the value of        the transfer function of the transmission channel for said        carriers and a substep of interpolating between said values in        order to deliver said first estimated transfer function.

The invention also provides a method of receiving a multicarrier signal,characterized in that it includes a method of estimating the transferfunction of the transmission channel of said multicarrier signal asdescribed above, and in that it further includes a step of compensatingthe modifications to which said multicarrier signal has been subjectedduring transmission on the basis of said second estimated transferfunction from which noise has been removed, in order to deliverestimated data.

According to other characteristics of this reception method:

-   -   it includes a plurality of prior steps of processing the        received signal in order to deliver said frequency-domain        digital signal; and    -   said multicarrier signal is a digital TV signal modulated by        orthogonal frequency-division multiplexing.

The invention also provides apparatus for estimating the transferfunction of a varying transmission channel for a multicarrier signalincluding a plurality of known reference items of information referredto as pilots, said apparatus receiving as input a frequency-domaindigital signal corresponding to the signal as received, digitized,synchronized, and transformed from a time base to a frequency base, andincluding a module for calculating the value of a first estimatedtransfer function of the transmission channel from the pilots containedin said frequency-domain digital signal, the apparatus beingcharacterized in that it further comprises:

-   -   a module for calculating the value of the autocorrelation        function of said first estimated transfer function enabling the        contribution of noise to be separated out;    -   a module for determining a noise-removal filter from said        autocorrelation function; and    -   a module for applying said noise-removal filter to said first        estimated transfer function in order to deliver a second        estimated transfer function of the transmission channel from        which noise has been removed.

According to other characteristics of this apparatus:

-   -   it is adapted to operate periodically; and    -   it is adapted to operate continuously.

The invention also provides a receiver unit for receiving a multicarriersignal, the unit being characterized in that it includes apparatus forestimating the transfer function of the transmission channel as definedabove, and in that it further includes a module for compensating theinfluence of the transmission channel in order to modify saidfrequency-domain digital signal as a function of said second estimatedtransfer function from which noise has been removed, in order to deliverestimated data.

According to other characteristics of this unit:

-   -   it further includes a preprocessing system adapted to receive        said multicarrier signal, and to deliver said frequency-domain        digital signal;    -   it is adapted to receive digital TV signals modulated by        orthogonal frequency-division multiplexing; and    -   it is mobile.

The invention also provides a computer program, characterized in that itcomprises program code instructions for executing the steps of themethod as defined above, when said program is executed on a computer.

Finally, the invention provides a programmed component, characterized inthat it comprises a logical configuration dedicated to executing thesteps of the method as defined above.

The invention will be better understood on reading the followingdescription, given purely by way of example and given with reference tothe accompanying drawings, in which:

FIG. 1 is a flow chart of the method of the invention; and

FIG. 2 is a functional diagram of apparatus for estimating the transferfunction of a transmission channel in accordance with the invention.

FIG. 1 is a flow chart of the method of the invention.

The method begins with a step 2 of radio transmission of a multicarriersignal identified by the letter A.

For example, the signal A corresponds to a digital television signalmodulated by orthogonal frequency-division modulation (OFDM).

The transmitted signal A includes a plurality of known items ofreference information referred to as pilots.

For example, in OFDM signal transmission, one in twelve of the carriersis a reference carrier and is referred to as a pilot carrier.

Thereafter, the method comprise a step 4 of receiving a signal Ycorresponding to the signal as transmitted and as modified by thetransmission channel.

This reception step 4 comprises a plurality of processing substepsassociated with the nature of the signal.

In the example of radio transmission of a digital signal with OFDMmodulation, this reception step comprises, for example, a substep ofconverting the analog signal into a digital signal, a substep oftransposition into base band, a substep of filtering and amplification,a substep of synchronization, and a substep of transformation from atime base to a frequency base.

This transformation substep corresponds to transforming the digitalsignal in the time domain into a representation thereof in the frequencydomain, implemented for example using a fast Fourier transform (FFT).

These substeps are conventional substeps in methods of receiving amulticarrier signal and they can be organized in different orders andthey can include other substeps depending on the nature of thetransmitted signal.

At the end of the reception step 4, the signal Y as delivered is adigital signal in the frequency domain representing the energydistribution of the received signal as a function of frequency.

Writing the real transfer function of the transmission channel as C, itcan be seen that the received signal Y is equal to the transmittedsignal A modified by the transfer function of the transmission channel,with the addition of noise written B.

Thus: Y=C.A+B.

The noise B is the result of the noise that is inherent to disturbancesin the transmission channel and also to the initial elements in thereception system, such as the tuner device or the amplifier, forexample.

Thereafter, the method comprises a step 6 of calculating the value of anestimated first transfer function C1 of the transmission channel, whichstep is performed in conventional manner.

During this step 6, the presence of the pilot carriers in the receivedsignal makes it possible to perform a substep of calculating themodifications to which these carriers have been subjected by comparingthe received signal Y and the transmitted signal A in respect of thepilot carriers.

It can thus be seen that for the pilot carriers, the value of thetransfer function C1 p is available, where:${C1p} = {\frac{Y}{A} = {C + \frac{B}{A}}}$

By interpolating between the values for these ratios C1 p relating tothe pilot carriers, a first estimate of the transfer function C1 for thetransmission signal is delivered.

Thereafter, the method comprises a step 8 of calculating the value ofthe autocorrelation function of the estimated first transfer functionC1.

The assumption that the transmission channel is variable implies therewill be variations in time in the transfer function of the transmissionchannel.

This autocorrelation function written R(C1) is calculated inconventional manner and serves to separate the contribution of noisefrom the transfer function of the signal.

This autocorrelation function R(C1) is used during a step 10 todetermine a noise-removal filter, written F.

The noise-removal filter is obtained from the autocorrelation functionusing a technique that is known in the state of the art and an exampleknown under the name Wiener filters is described on pages 446 to 453 ofthe book “Probability, random variables, and stochastic processes”, byAthanasios Papoulis.

Naturally, filters other than Wiener filters could be used fordetermining the noise-removal filter from the autocorrelation functionR(C1).

During a step 12, the noise-removing filter F is applied, e.g. by aconvolution product calculation, to the first estimated transferfunction C1 of the transmission channel, thereby obtaining a secondestimated transfer function of the transmission channel C2 from whichnoise has been removed.

Thus, C2=F*C1.

Thereafter, during a step 14, this second estimated transfer function C2from which noise has been removed is used to correct the received signalin order to obtain an estimate A1 of the transmitted data, such that:${A1} = \frac{Y}{C2}$

For a transmission channel that is varying, the method needs to beimplemented periodically, and it is advantageously performedcontinuously on processing time windows of duration that corresponds tothe duration of an OFDM symbol, for example.

When the method is implemented periodically, it is appropriate to definean operating period during which the variations in the transmissionchannel can be considered as being slow and continuous.

The method described can be implemented, for example, by a computerprogram or by a programmable component, having a logical configurationthat is specially modified to enable calculation to be performed.

The program may also be stored in a non-volatile memory so as to beexecuted on a request coming from a microcontroller or a microprocessor,such as a microcontroller or a microprocessor integrated in a digital TVsignal decoder, for example.

FIG. 2 shows a receiver unit including apparatus for estimating thetransfer function in accordance with the invention.

In the figure, there can be seen a transmitter 20 such as a radiotransmitter for a digital TV signal A that is OFDM-modulated and that istransmitted over the transmission channel as modelled by the transferfunction C.

The signal is received by the receiver unit 22 using an antenna 24.

The received signal is then injected into a preprocessing system 26which comprises, for example: an analog-to-digital converter 27, asynchronization module 28, and a module 29 for transforming from a timebase to a frequency base using a fast Fourier transform (FFT), themodule 29 implementing step 4 of the method as described above.

The preprocessing system 26 is connected at its output to a module 31for calculating a first estimated transfer function by implementing step6 of the method.

The module 31 has its output connected to a module 32 for calculatingthe value of the autocorrelation function, and its output is connectedto a module 34 for determining a noise-removal filter.

These modules respectively implement steps 8 and 10 of the method of theinvention.

The apparatus 30 for estimating the transfer function then comprises amodule 36 for applying the noise-removal filter as determined by themodule 34 to the estimated transfer function as calculated by the module31, so as to implement step 12 of the method, and deliver a secondestimated transfer function from which noise has been removed.

Thereafter, the receiver unit 22 comprises a delay module 38 fordelaying the signal delivered by the preprocessing system 26 by theduration that is required for estimating the second channel transferfunction from which noise has been removed. Finally, the unit 22comprises a compensation module 40 implementing step 14 of the methodand enabling the modification due to the transmission channel that occurin the delayed received signal to be compensated by using the secondestimated transfer function C2 from which noise has been removed.

Thus, in operation, the signal A is transmitted from the transmitter 20over the transmission channel C in order to be received by the antenna24.

The received signal is digitized, synchronized, and transformed from atime base into a frequency base in the preprocessing system 26 so as todeliver the corresponding frequency-domain digital signal Y.

The signal Y is inserted into the apparatus 30 for estimating thetransfer function.

The module 31 then determines what changes have occurred in the pilotcarriers, and by interpolation delivers a first estimated transferfunction C1 for the transmission channel. This estimated transferfunction C1 is injected into the module 32 which calculates theautocorrelation function written R(C1).

Using the autocorrelation function, the module 34 for determining thefilter uses Wiener's filter equations, for example, to determine thenoise-removal filter F.

The module 36 then applies the filter F to the first estimated transferfunction C1 in order to deliver the second estimated transfer functionC2 from which noise has been removed.

In parallel with the calculation for estimating the transfer function,the frequency-domain digital signal Y is introduced into the delaymodule 38, after which the delayed signal Y is introduced into themodule 40 which processes this signal as a function of the secondestimated transfer function C2 from which noise has been removed inorder to deliver the estimated data A1.

Thereafter, in the context of receiving a digital TV signal, theestimated data A1 is decoded and played back.

It can thus be seen that the apparatus for estimating the transferfunction of a transmission channel in accordance with the inventionenables the estimated transfer function of this transmission channel andfrom which noise has been removed to be calculated for a transmissionchannel that varies over time.

In particular, such apparatus is particularly suitable for use in amobile receiver unit.

For example, such a receiver unit is integrated in a mobile handset forreceiving terrestrial digital TV signals.

Furthermore, when implementing diversity reception in which a pluralityof transmission channels and corresponding reception systems are used,each reception system can be fitted with apparatus of the invention,with all of the estimated data subsequently being processed by a decoderhaving inputs that are weighted using methods that are conventional indiversity reception.

1. A method of estimating the transfer function of a varyingtransmission channel for a multicarrier signal including a plurality ofknown reference data items referred to as pilots, said method beingimplemented from a frequency-domain digital signal corresponding to thesignal as received, digitized, synchronized, and transformed from a timebase to a frequency base, and including a step of calculating the valueof a first estimated transfer function for the transmission channel onthe basis of the pilots contained in said frequency-domain digitalsignal, wherein said step of calculating a first estimated transferfunction comprises a substep of calculating the modifications to whichthe pilots have been subjected in order to define the value of thetransfer function of the transmission channel for said carriers and asubstep of interpolating between said values in order to deliver saidfirst estimated transfer function, and wherein the method furthercomprises: a step of calculating the value of the autocorrelationfunction of said first estimated transfer function enabling to separateout the contribution from noise; a step of determining a noise-removalfilter from said autocorrelation function; and a step of applying saidnoise-removal filter to said first estimated transfer function in orderto deliver a second estimated transfer function of the transmissionchannel from which noise has been removed.
 2. A method according toclaim 1, wherein the method is implemented periodically.
 3. A methodaccording to claim 1, wherein the method is implemented continuously. 4.A method of receiving a multicarrier signal, the multicarrier signalincluding a plurality of known reference data items referred to aspilots, said method being implemented from a frequency-domain digitalsignal corresponding to the signal as received, digitized synchronized,and transformed from a time base to a frequency base, the methodcomprising: a step of calculating the value of a first estimatedtransfer function for the transmission channel on the basis of thepilots contained in said frequency-domain digital signal, wherein saidstep of calculating the value of a first estimated transfer functionincludes: a substep of calculating the modifications to which the pilotshave been subjected in order to define the value of the transferfunction of the transmission channel for said carriers; and a substep ofinterpolating between said values in order to deliver said firstestimated transfer function; a step of calculating the value of theautocorrelation function of said first estimated transfer functionenabling to separate out the contribution from noise; a step ofdetermining a noise-removal filter from said autocorrelation function; astep of applying said noise-removal filter to said first estimatedtransfer function in order to deliver a second estimated transferfunction of the transmission channel from which noise has been removed;and a step of compensating the modifications to which said multicarriersignal has been subjected during transmission on the basis of saidsecond estimated transfer function from which noise has been removed, inorder to deliver estimated data.
 5. A method of receiving a multicarriersignal according to claim 4, wherein the method includes a plurality ofprior steps of processing the received signal in order to deliver saidfrequency-domain digital signal.
 6. A method according to claim 1,wherein said multicarrier signal is a digital TV signal modulated byorthogonal frequency-division multiplexing.
 7. An apparatus forestimating the transfer function of a varying transmission channel for amulticarrier signal including a plurality of known reference items ofinformation referred to as pilots, said apparatus receiving as input afrequency-domain digital signal corresponding to the signal as received,digitized, synchronized, and transformed from a time base to a frequencybase, and including a module for calculating the value of a firstestimated transfer function of the transmission channel from the pilotscontained in said frequency-domain digital signal, wherein said modulefor calculating the value of a first estimated transfer function isadapted to determine the modifications to which the pilots have beensubjected and to perform interpolation between said values in order todeliver said first estimated transfer function, the apparatus furthercomprising: a module for calculating the value of the autocorrelationfunction of said first estimated transfer function enabling thecontribution of noise to be separated out; a module for determining anoise-removal filter from said autocorrelation function; and a modulefor applying said noise-removal filter to said first estimated transferfunction in order to deliver a second estimated transfer function of thetransmission channel from which noise has been removed.
 8. An apparatusaccording to claim 7, wherein the apparatus is configured to operateperiodically.
 9. An apparatus according to claim 7, wherein theapparatus is configured to operate continuously.
 10. A receiver unit forreceiving a multicarrier signal, the multicarrier signal including aplurality of known reference items of information referred to as pilots,said receiver unit receiving as input a frequency-domain digital signalcorresponding to the signal as received digitized, synchronized, andtransformed from a time base to a frequency base, and including a modulefor calculating the value of a first estimated transfer function of thetransmission channel from the pilots contained in said frequency-domaindigital signal, wherein said module for calculating the value of a firstestimated transfer function is adapted to determine the modifications towhich the pilots have been subjected and to perform interpolationbetween said values in order to deliver said first estimated transferfunction the receiver unit further comprising: a module for calculatingthe value of the autocorrelation function of said first estimatedtransfer function enabling the contribution of noise to be separatedout; a module for determining a noise-removal filter from saidautocorrelation function; a module for applying said noise-removalfilter to said first estimated transfer function in order to deliver asecond estimated transfer function of the transmission channel fromwhich noise has been removed; and a module for compensating theinfluence of the transmission channel in order to modify saidfrequency-domain digital signal as a function of said second estimatedtransfer function from which noise has been removed, in order to deliverestimated data.
 11. A receiver unit according to claim 10, wherein thereceiver unit further includes a preprocessing system adapted to receivesaid multicarrier signal, and to deliver said frequency-domain digitalsignal.
 12. A receiver unit according to claim 10, wherein the receiverunit is configured to receive digital TV signals modulated by orthogonalfrequency-division multiplexing.
 13. A receiver unit according to claim10, wherein the receiver unit is configured to be mobile.
 14. A computerprogram product residing on a computer readable medium having aplurality of instructions stored thereon which, when executed by theprocessor, cause that processor to: calculate, from a frequency-domaindigital signal corresponding to a multicarrier signal as received,digitized, synchronized, and transformed from a time base to a frequencybase, the value of a first estimated transfer function for a varyingtransmission channel for said multicarrier signal on the basis of pilotscontained in said frequency-domain digital signal, wherein calculatingthe value of the first estimated transfer function includes: calculatingthe modifications to which the pilots have been subjected in order todefine the value of the transfer function of the transmission channelfor said carriers; and interpolating between said values in order todeliver said first estimated transfer function; calculate the value ofthe autocorrelation function of said first estimated transfer functionenabling to separate out the contribution from noise; determine anoise-removal filter from said autocorrelation function; and apply saidnoise-removal filter to said first estimated transfer function in orderto deliver a second estimated transfer function of the transmissionchannel from which noise has been removed.
 15. A program component,including a logic configuration, configured to: calculate, from afrequency-domain digital signal corresponding to a multicarrier signalas received, digitized, synchronized and transformed from a time base toa frequency base, the value of a first estimated transfer function for avarying transmission channel for said multicarrier signal on the basisof pilots contained in said frequency-domain digital signal, whereincalculating the value of the first estimated transfer function includes:calculating the modifications to which the pilots have been subjected inorder to define the value of the transfer function of the transmissionchannel for said carriers; and interpolating between said values inorder to deliver said first estimated transfer function; calculate thevalue of the autocorrelation function of said first estimated transferfunction enabling to separate out the contribution from noise; determinea noise-removal filter from said autocorrelation function; and applysaid noise-removal filter to said first estimated transfer function inorder to deliver a second estimated transfer function of thetransmission channel from which noise has been removed.